The Smaller Oligomers

51V chemical shifts for aqueous peroxo and hydroxamido divanadium(V) complexes 

3 These complexes were observed at high total vanadate (80 mmol/L) and total ligand (80 mmol/L) concentrations where they are minor species [2]. b See arguments in text concerning charge state of this complex [14]. 

Although linear vanadate oligomers, up to V6, have been identified as minor products in aqueous solution, little is known of their chemistry. Presumably, they will undergo monodentate and other reactions that involve only one vanadium center similar to those observed for V2. A V3L2 complex has been found for an a-hydroxy- 

In addition to the linear oligomers, cyclic derivatives (V33, V44, V/ ) are also known (see Section 2.2). The latter two are readily formed and are ubiquitous components of aqueous solutions. These compounds are relatively inert to complex-ation reactions that preserve their cyclic structure rather, the equilibria shift towards products of other vanadium stoichiometry. Presumably, one way to generate complexes of these oligomers would be to protonate one or more oxygens so that the reactivity is increased. However, neither protonated V4 nor V5 has been identified in solution, nor has there yet been any indication from studies in alcoholic solution that an alkoxo group can replace an oxo ligand, as might be expected if protonation occurs. 

Under anhydrous conditions, tetranuclear vanadium clusters such as [V404 (0CH2)3CCH3 3(0C2H5)3] can be generated in alcoholic solutions with l,l,l-tris(hydroxymethyl)ethane. The coordination does vary somewhat with the alcohol utilized in the preparation, but all retain the tetranuclear structure. Each vanadium nucleus in the complex has octahedral coordination. The clusters are not stable to hydrolysis and decompose in chloroform solution in the presence of just a few equivalents of water [5], 

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It was possible to obtain an excellent fit of Eq. (8) with Eq. (9) to the combined M and T dependence of D and extract of value VE - 13.7 0.2 cm3/mol, slightly more in a similar polymer having one terminal —OH group. The constants P and fg had to be fitted as well, the latter assuming the value fg = 0.023 0.001. This interpretation of melt diffusivity was confirmed in separate experiments in which diffusion of the smaller oligomers was measured in a high molecular weight cispolyisoprene host (see below). Data and fitted theory in the melts are shown in Fig. 4. 

A template effect is concluded from the observation that CsOH favours the formation of the larger macrocycle 11b in the case of x = 2 while the smaller oligomer 11a is predominant with NaOH as catalyst . For x = 0 a cyclic dimer is not formed, but again the trimer 10b is formed with NaOH and the tetramer 10c with CsOH as base . 

Once these nuclei have consumed all the smaller oligomer species, the resulting colloidal particles remain in solubility equilibrium with monomer. However, if the concentration is more than a few thousand parts per million (0.6% or 0.1 M SiOj) the particles soon aggregate into strings or branched networks of microgel. 

In order to transfer the attractive solution-processing properties of polymers to oligomers, the smaller oligomers can be rendered soluble either by suitable chemical modifications, such as addition of alkyl side-chains in a similar fashion as the poly(3-alkylthiophenes) [34-37] (see Fig. 2b,c) or else they can be blended within a soluble polymer [10-12, 14, 15] or chemically grafted as pendant side-chains on a polymer backbone [16, 17], as shown in Fig. 3. 

The reactivity of sulfur clearly depends sensitively on the molecular ctimplexity of the reacting species. Little systematic work has been done. Cyc/<7-Ss is obviously less reactive than the diradical catenas, and smaller oligomers in the liquid or vapour phase also complicate the picture. In the limit atomic sulfur, which can readily be generated photolytically, is an extremely reactive specie.s. As with atomic oxygen and the various... 

AH substrates (varying between 0.018 and 0.05% w/v) were incubated in 50 mM sodium acetate buffer pH 5.0, containing 0.01% w/v sodium azide, at 40 °C for 24 h. RGO s were treated with 2.6 pg RG-gaiacturonohydrolase per mg substrate. When RGO s were sequentially treated with the exo-enzymes to form smaller oligomers, the RG-galacturonohydrolase and the RG-rhamnohydrolase were used in amounts between 2.4 and 2.8 pg and between 9 and 18 pg per mg substrate respectively. RGO s were incubated with 0.18 pg RG-hydrolase and with 0.42 pg RG-lyase per mg substrate. Subsequent incubation of the RG-hydrolase/RG-lyase digest with the exo-enzymes was carried out with 6 pg of RG-galacturonohydrolase and with 16 pg RG-rhamnohydrolase per mg substrate. 

Second, the polymer needs to be broken down into small fragments. Microorganisms excrete extracellular enzymes that cleave the polymeric chains [4]. This enzymatic cleavage reaction, on the one hand, needs functional sites within the polymer backbone where the enzymes can catalyze the cleavage of chemical bonds. On the other hand the polymeric chain needs to be flexible enough that the chain can enter the catalytic site of the enzyme. In most cases, the chemical reaction catalyzed by the exo-enzymes is a hydrolysis process that converts the polymer chain into smaller oligomers and monomers [4]. 

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